The present invention relates to electrical motors, and more particularly to spindle motors used to controllably rotate discs in the course of storing data on the discs or retrieving previously stored data from the discs.
Disc drives, in which one or more discs are supported to rotate on a spindle, are frequently employed in connection with storing magnetic data. Typically a magnetic transducing head is movable radially of the disc which, in combination with disc rotation, permits a selective positioning of the transducing head with respect to the recording surface of the disc. Similarly in connection with optical recording, a laser beam is movable generally radially of a rotating disc for writing information onto the disc, or for reading previously written information. In either case, satisfactory operation of the disc drive depends upon rapidly and smoothly accelerating the hub and discs to a desired rotational speed, maintaining the desired speed, then decelerating when it is desired to stop the discs. To this end, the spindle motor should have a high starting torque, and further should have minimal variance in torque from one angular location to another, i.e. a minimum torque ripple.
It is known that torque ripple effects can be reduced by introducing either radial or longitudinal asymmetry into the magnet of the spindle motor. For example, small magnet may be mechanically coupled or bonded to primary magnets with opposite poles of the respective magnets adjacent one another. Material can be cut away or otherwise removed from an originally symmetrical magnet to produce selective asymmetrical features. Yet another approach is to use non-functioning magnetic material to shunt magnetic flux away from the coils of the spindle motor stator. The current provided through various coils in the stator or rotor may be intermittently switched or otherwise varied, introducing fluctuations in the magnetic field intended to counteract torque ripple. These approaches, however, reduce the efficiency of the magnet and/or stator and thus degrade performance of the spindle motor. Further, they add to the expense of the motor in terms of added materials or cost of assembly.
Therefore, it is an object of the present invention to provide a spindle motor in which torque ripple is reduced without requiring either radial or longitudinal asymmetry in either the rotor or stator structures.
Another object is to provide a spindle motor in which an annular magnet is selectively magnetized to provide magnetic flux pattern across an air gap between the magnet and stator, particularly suited to reduce torque ripple.